This invention relates to a laminated article comprising a core portion formed from sheets of mica paper. The sheets are composed primarily of synthetic mica platelets, and are impregnated with an organic resin. A plurality of such impregnated paper sheets are stacked, and the stack consolidated by pressing, to form the core.
Synthetic mica materials, the processing of such materials to form gels, and the use of such gels to produce films, papers, boards, and the like, are disclosed in U.S. Pat. No. 4,239,519 (Beall et al.)
U.S. Pat. No. 4,559,264 (Hoda et al.) discloses a substrate in which a plurality of synthetic mica papers are laminated together. Each paper is composed of overlapping, ion-exchanged platelets of a synthetic, lithium and/or sodium, water-swelling mica, and is impregnated with an organic resin. A laminated stack of such papers has a planar, isotropic coefficient of thermal expansion in the x-y plane which may be less than 100.times.10.sup.-7 /.degree.C. over a range of 25.degree.-300.degree. C.
When the substrate is used in forming an electronic component, it may be provided with conductive connections by subtractive or additive circuitry processes. For this purpose, a conductive metal foil, usually copper, is attached over a face on the core. It is a prime requisite that such copper foil strongly adhere to the core. This property, known as copper peel strength, is measured in lbs/inch, and is the force required to separate the copper foil from the core face.
Another property of considerable interest is insulation resistance. This is measured in ohms and is the resistance of the core surface to passage of electric current. Moisture pickup is also a concern. While many tests are available, one commonly used is the weight gain in percent of a body after two hours immersion in boiling water. Dieletric constant and loss tangent are also commonly specified properties.
A very low copper peel strength was observed when a copper foil was assembled directly against the face of a synthetic mica sheet stack and the assembly consolidated. To correct this situation, a glass fiber mat or fabric was interposed between the metal and the stack to act as a facing. This does tend to improve peel strength but, obviously, introduces an added step and a non-micaceous material. Both of these would desirably be avoided.
There is a constant desire to increase the potential or maximum operating temperature of a component. Accordingly, a transition temperature (Tg) as high as 200.degree. C. may be specified for a core. Such temperatures were very difficult to attain with previously used epoxy resins.